1. Field of the Invention
The present invention is directed to an integrated optical modulator for modulation of optical signals in dielectric wave guides which are disposed on a substrate and with at least one of the wave guides consisting of electro-optical material whose propagation characteristic or constant is changeable by the application of a potential on electrodes disposed on the substrate.
2. Prior Art
If two or more dielectric, monomode wave guides are disposed in parallel relationship on a substrate for a sufficient length, a periodic exchange of energy may take place between the two wave guides. In the case of these two coupled wave guides, the greatest interaction between the two wave guides occurs when the phase velocity and the field distribution in the wave guides are matched and the coupling structure is loss free. If at the beginning of the coupling length all of the energy is conducted in one wave guide, a part of the energy, which under the above-mentioned optimum conditions is the entire energy, is at periodic intervals coupled to the second wave guide and then coupled or conducted back to the first wave guide.
If one produces one or both wave guides from an electrooptical material, an electro-optical amplitude modulator or switch can be produced with this type of structure. For this purpose, the coupling length is dimensioned to be such that when no electrical field is applied, the entire or full energy is coupled to the second wave guide. As a result of an application of an electrical field to one or both of the wave guides, it is then possible to make the phase velocity in the two wave guides to be different and thus to achieve a modulation of the coupled energy. When the two wave guides have a phase difference of 180.degree. over the entire coupling length, the coupled energy is zero.
A practical embodiment of such a modulator is described in an article by H. F. Taylor, "Optical Switching and Modulation in Parallel Dielectric Waveguides", Journal Applied Physics, vol. 44, No. 7, July 1973, pages 3257-3262. The modulator described in this article has a pair of wave guides arranged next to each other on a substrate, which wave guides are simultaneously modulated. The coupling strength is determined by the spacing between the two wave guides and the difference in the index of refraction between the core of each wave guide and the wave guide material surrounding the core. However, the permissible tolerances for the spacing between the two wave guides are extremely narrow. For example, with a coupling length of 4 mm, a deviation of 1000 angstroms in the spacing between the pair of wave guides can lead to a change of approximately 20% in the amount of coupled energy.